UCRL 640 Rev cy 2
UNIVERSITY OF CALIFORNIA
TWO-WEEK LOAN COPY
This is a Library Circulating Copy which may be borrowed for two weeks. For a personal retention copy, call Tech. Info. Diuision, Ext. 5545
BERKELEY, CALIFORNIA DISCLAIMER
This document was prepared as an account of work sponsored by the United States Government. While this document is believed to contain correct information, neither the United States Government nor any agency thereof, nor the Regents of the University of California, nor any of their employees, makes any warranty, express or implied, or assumes any legal responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by its trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof, or the Regents of the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof or the Regents of the University of California. UCRL~640 Rev o Unclassified Distribution /
•• UNIVERSITY OF CALIFORNIA
Radiation Laboratory
Contract No 0 W-7405-eng~48
POLONIUM ISOTOPES PRODUCED WITH HIGH ENERGY PARTICLES
... D. G. Karraker and D. Ho Templeton
September 25 1 1950
Berkeley1 California Unclassified :Distribution·
'"( INSTALLATION Number of Copies
Argonne National Laboratory 8 Armed Forces Spe_cial Weapons Project 1 Atomic Energy Commission =Washington 2 Battelle Memorial Institute 1 Brush Beryllium Company 1 Brookhaven National Laboratory 4 Bureau of Medicine and Surgery 1 Bureau of Ships 1 Carbide & Carbon Chemicals Division (K-25 Plant) 4 Carbide & Carbon Chemicals Division (Y-12 Plant) 4 Chicago Operations :office 1 ColUmbia University (Jo Ro Dunning) 1 Columbia University (G& Failla) 1 Dow Chemical Company 1 Ho K.. Ferguson Company 1 General Electric Company; Richland 3 Harshaw Chemical Corporation 1 Idaho Operations Office 1 Iowa state College . 2 Kansas City Operations Branch 1 Kellex Corporation 2 Knolls Atomic Power Laboratory 4 Los Alamos ScientU.'ic Lab.oratory 3 Mallinckrodt Chemical Works 1 Massachusetts Institute of Technology (Ao Gaudin) 1 Massachusetts Institute of Technology (Ao Ra Kaufmann) 1 Mound Laboratory 3 National Advisory Committee for Aeronautics 1 National Bureau of Standards 3 NavaJ. Medical Res.earch Institute 1 Naval Radiological Defense Laboratory 2 New Brunswick Laboratory · 1 New York Operations Office 3 North American Aviation~ Inca 1 Oak Ridge National Laboratory , 8 Patent Branch - WaShington 1 RAND Corporation 1 Sandia Corporation 2 Santa Fe Operations Office 2 Sylvania Electric Products, Inco 1 Technical Information Division (Oak Ridge) 15 Armament Division,p Deputy for Research and Development 1 (Capt .. Glenn Davis) . Assistant for Atomic Energy~ Deputy Chief of Staff 1 (Colo Robert Eo Greer) · Chief of Documents and Disseminations Branch (Col. J. E. Mallory) 1 USAF Assistant for Research Director of Research and. Develtp- _
ment 8 Deputy Chief of Staff (Col~ B. G. Holzman) 1 Unclassified Distribution
~2a-
INSTALLATION Number of Copies,
Electronic Systems Division (Mro Eo Co Trafton) 1 Chief of Scientific Advisors (Dr. Theodore von Karman) 1 USAF. Eglin Air Force Base (Major Ao Co -Field) 1 USAF, Kirtland Air Force Base (Col. Marcus F. Cooper) 1 USAF, Maxwell Air Force Base (Col. F o N. Moyers) 1 USAF, NEPA Office 2 USAF, Offutt Air Force Base· (Col. H. R. Sullivan, Jr.) 1 USAF Surgeon General, Medical Research Division (Col. A.P. Gagge) 1 USAF, Wright-Patterson Air Force Base (Rodney Nudenberg) 1
U0 s. Army, Atomic Energy Brw.ch (Lt. Col. A. W. Betts) 1 u. s. Army, Army Field Forces (Captain Jrunes Kerr) 1 u. s. Army, Commanding General, Chemical Corps 1 Technical Command (Col. John A. MacLaughlin thru Mrs. Georgia s. Benjamin) u. s. Ar.my, Chief of Ordnance (Lt. Col. A. R. Del Campo) 1 u. s. Army~- Commanding Officer~ Watertown Arsenal ( 1 (Col. Carroll H. Deitrick) - Uo s. Army, Director of Operations Research (Dr. Ellis Johnson) 1 U. So Army, _Office of Engineers (All en 0' Leary) 1 u. s. Ar.my, Office of the Chief Signal Officer 1
(Curtis T~ Clayton thruMaj. George c. Hunt) io' u. s. Army, Office of the Surgeon General (Col. w. s. Stone) 1 u. s. Geological Survey (T. B. Nolan) 2 u. s. Public Health Service 1 University of California at Los Angeles 1 University of California Radiation Laboratory 5 University of Rochester 2 University of Washington 1 Western Reserve University 2 Westinghouse Electric Company 4 R. F. Bacher (California Institute of Technology) 1 Cornell University 1
Total 140
Information Division Radiation Laboratory Univ. of California Berkeley, California- . UCRL-640 Revised Page 3
POLONIUM. ISOTOPES PRODUCED WITH HIGH ENERGY. PARTICLES * D. G. Karraker and D. H. Templeton Radiation Laboratory and Department of Chemistry University of Californiaj Berkeleyj California September 25j 1950
ABSTRACT We have investigated four new isotopes of polonium produced from bismuth and lead by high-energy spallation and have studied their decay productsj which include three new lead and bismuth activities. The mass assignments are made by the identification of known activities as decay products. Some of the proper- ties of these new isotopes are as follows:
Isotope Half-life Mode of Energy in Mev .. Decay Alpha Gamma Rays Particles
p 0 205 1.5 hr. EC,c 5.22.!: 0.10 p 0 204 3.8 hr. EC}n 5.37 :!: 0.02
p0 203 47 min. EC p + 0 202 52 min. EC5a. 5.59 0.03
Bi205 14.5 days EC 0.431, 0.527 0.550, 0.746 1.84 95 min. EC 25 min. EC
*This work was done under the auspices of the U. S. Atomic Energy Commission. UCRL-640 Revised Page 4
POLONIUM ISOTOPES PRODUCED WITH HIGH ENERGY PARTICLES
D. G. Karraker and D. H. Templeton Radiation Laboratory and Department of Chemistry University of California, Berkeley, California
INTRODUCTION
Polonium has eight isotopes, beta stable or with neutron excess, which occur in the natural radioactive series or the artificial neptunium series.
Cyclotron-induced transmutations now make possible the production of neutron deficient polonium isotopes. Templeton, Howland, and Perlman1 have produced by this means the three lsotopes. Po 206 , Po 207 , and Po 208 , and Kelly and Segre"2 have found Po209. The present work was undertaken to extend our knowledge of polonium to the even lighter isotopes which can be made by the 184-inch
Berkeley cyclotron. We have observed the radioactivities of the next four 205 204 203 202 isotopes, Po , Po , Po , and Po • Experiments designed to establish the decay products and mass assignments of these radioactivities led to the discovery of two radioactive species of bismuth and one of lead. Our work was greatly aided by a parallel investigation of bismuth and lead isotopes carried out in this same laboratory by Neumann and Perlman, who have already 3 reported their results.
EXPERIMENTAL METHODS
In most of the experiments a target of natural lead or bismuth was bombarded with particles accelerated in the 184-inch Berkeley cyclotron.
Helium ions with lead, or protons or deuterons with bismuth produced good
1Templeton, Howland, and Perlman, Phys. Rev. 72, 758 (1947).
2E. L. Kelly and E. Segre, Phys. Rev. 75, 999 (1949). 3H. M. Neumann and I. Perlman, Phys. Rev. 78, 191 (1950). UCRL-640 Revised Page 5
yields of the polonium isotopes of interest, but also substantial amounts of
other spallation and fission products. Particles of various energies were
used to improve the relative yield of the isotope desired in a particular
experiment. The target was made of metal in the form of strips 0.5 to 2 mm
thick; or, when speed in. chemical separation was required, bismuth oxide was
used. This oxide can be dissolved in acids more rapidly than the metal.
All samples were separated chemically before measurements were made.
Polonium was separated from other elements by a procedure using tellurium, with
hold-back carriers added for thallium and lead. The tellurium was reduced
to the element with stannous chloride, carrying with it the polonium and noble
metals. The tellurium was dissolved and precipitated with sulfur dioxide,
carrying again the noble metals$ but leaving a carrier-free solution of polon1um,
with 85-95 percent yield. For further purification, polonium was extracted from
6N HCl into a mixture of 20 percent tributyl phosphate and 80 percent dibutyl
ether. The extraction coefficient for polonium between the organic and acid
layers is about 110. Lead and bismuth daughter activities were removed quanti- tatively by washing the organic layer with 6li HCl, and purified by precipitation== bismuth as BiOCl and lead as Pbso or PbCro . 4 4 Thallium activities were separated by oxidation of thallium to the thallic
state with potassium permanganate, followed by the extraction of thallic chloride
with diisopropyl ether saturated with HCl. Occasionally the thallium activities
were further purified by evaporation of the ether, reduction of the thallium
with hydrogen peroxide, and precipitation of the thallium as Tl2PtC16 in the presence of lead and bismuth hold=back carriers .
... UCRL-640 Revised Page 6
Generally, the decay curve of the polonium Geiger activities is so complex that resolution is ambiguous. To avoid this difficulty, the half-lives were determined by separation of the daughter activities at a sequence of equal time intervals, the interval corresponding approximately to the half-life of the parent. The activity of the daughter at time ~ is given by~ ! 2
where Q2 is the counting efficiency of the daughter, including the geometr,y factor, )l2 andA1 are the decay constants of daughter and parent, respectively; and N is the number of atoms of parent at t = 0. At the start of the time -o ' - interval, the parent is purified chemically so that the additional term in the general formula does not entE!r the expression. It will be observed that~ if the time t is the same for all periods of growth of the daughter before separation from the parent, the exponential terms become a constant factor, and ! 2 is proportional to the disintegration rate (~1N0 ) of the pa~ent at the beginning of the growth period. Thus, if one plots the logaritr~ of the initial activity of each daughter fraction against the time of separation, the slope of the line will correspond to the half-life of the parent.
In the identification of alpha-decay daughters, the thallium electron capture daughter of the lead alpha-dec~ daughter was usually separated for measurement, rather than the lead activity itself. This procedure was followed since a greater degree of purity was attained in the thallium sepa- rations than in the lead separations; furthermore, the decay curve of the thallium activities is simpler than the dec~ curve of the lead activities, since no similar half-lives are found and no daughters are growing. UCRL-640 Revised Page 7 Isotopes of interest in this paper are shown in Table I, where isotopes
enclosed in parentheses are isotopes identified by this work, and stable isotopes are shaded. Table Ia,b
198 199 200 201 202 203 204 205 206 207 208
Po (52m (47m (3 .8h (1.5h 9d 5.7h 3y EC,a) EC) EC,a) EC,a) EC,a EC,a a Bi (95m 12h 12h (14.5d 6.4d long EC) EC EC EC) EC Pb (25m 80m 18h 8h long EC) EC EC EC · Tl
.. Hg
aG. T. Seaborg and I. Perlman, Rev. Mod. Phys. 20, 585 (1948). bH. M. Neumann and I. Perlman, Phys. Rev. 78, 191 (1950).
3 .$-Hr. Po204 The alpha-decay of polonium produced by high energy irradiation shows
periods of about 45 minutes, 1.5 hours, and 4 hours, in addition to longer periods previously identified. To determine the mass assignments of these isotopes, their daughters were identified by the method sketched above. A pure sample of mixed polonium activities was separated from the bombarded target (elapsed time, 2 hours) and the daughter activities separated at intervals of an hour. The decay of the purified bismuth fraction showed a 95-minute period (which will be discussed later), a 12-hour period, and a longer period of 6 to 14 days, in low intensity. Fig. 1 is a plot of the UCRL-640 Revised Page 8
il
Fig. 1. Data showing the genetic relationship of 20 3.8-hr. Po 204 and 12-hr. Bi 4. UCRL-640 Revised Page 9
0 I I I I
~ 0
>- 1- ;: 1000 t- o ~
24 30 36 42 HOURS MU -829
Fig. 1 UCRL-640 Revised Page 10
decay of the bismuth fractions for three of the separated samples where the ' 12-hour period has been extrapolated back to the time of separation. (Only three of the bismuth decay curves are shown, to avoid crowding. Extrapolated points from the decay curves of the other fractions are plotted at the time of separation.) It will be seen from Fig. 1 that the yields at the time of sepa- ration determine the half-life of the parent activity of the 12-hour bismuth as 3.8 hours, in agreement with the 3.8-hour value obtained from alpha-decay curves. It will be seen from Table I that the establishment of a 12-hour bismuth
a·ctivity as the dau~ter of the 3.8-hour polonium does not give an unambiguous
mass number. A similar experiment l'la.S done to identify the alpha-decay daughter and fix the mass number. As before, a large sample of purified polonium activi- ties was prepared, from which the bismuth and lead daughter activities were separated at 4-hour intervals. The bismuth activity could be ignored, as none of the bismuth daughters are alpha-emitters, and Po200 is presumed to be too . 200 short to produce any of the Tl · through an electron capture chain. The lead alpha-daughter was allowed to grow its thallium daughter for about 18 hours; then the thallium was separated. Since the lead fraction had been separated at 4-hour intervals, the activity of the alpha-decay daughter of the 3.8-hour polonium would be decreased by approximately a factor of two between each of the fractions. The time allowed for the thallium to grow was the same for each fraction, so the activity of the lead alpha-daughter will be directly ·proportional to the activity of its thallium daughter. The principal, and virtually the only activity found was the 27-hr. Tl200 • The results are shown in Table II. UCRL-640 Revised Page 11
Table II
Fraction 27-hr. Tl, counts/minute Observed Calculated
1 3280
2 1740 1600 3 845 780
These experiments have fixed the 3.8-hour polonium at mass 204. Its alpha-decay daughter is the 18-hr. Pb200 which was shown by Neumann and Perlman3 to be the parent of Tl200 • The electron capture daughter is Bi 204, which has been shown4 to decay both to Pb204·and to Pb204m. These decay relations are:
Pb204m 68 min. 1I.T. p 204 20 0 EC- Bi204 _.....;;;;...;.__:__EC _ _.!> Pb 4 3.8 hr. > 12 hr. stable 1· Pb200 EC Tl200 EC Hg200 18 hr. !> 27 hr. stable
We have confirmed that Po204 gives rise to the 68-min. Pb204m, by an experiment which is described below. Thus these assignments, Which are based on the excitation curve for the formation of 27-hour t~allium by alpha irradiation of gold5 are an independent check on the assignment4 of Pb204m. This isomer is of great interest because of its unique properties as a long-lived highly
4Templeton, Howland, arid Perlman, Phys. Rev. 72, 766 (1947). 5 Orth, ~rquez, Heiman, and Templeton,.Phys. Rev. 75, 1100 (1949). UCRL=640 Revised Page 12 excited state of an even-even nucleus. The energy of the alpha-particles from Po204 was determined as 5,37 ! 0,02 6 Mev by pulse analysis (Fig. 2). The ratio of electron capture disintegrations to alpha disintegrations has not been determined with accuracr,r. It is estimated as approximately 100 on the assumption that the cross section for the (p 9 6n) reaction on bismuth is 0.1 barn at its maximumj which is at 70 to 80 Mev.
The discover,y of two 12=hour bismuth isotopes forced a reconsideration of the mass assignment of Pb204mj which was based in part on the observation that the 68-minute lead is a daught.er of a 12=hour bismuth, The experiments4 which showed that this is the casej and that 52~hr. Pb203 is not the daughter of the same 12-hour bismuthj were done with bismuth from the alpha irradiation of thallium at an energy which did not produce Bi203 • Had these experiments been done on the bismuth produced r'rom leadj both daughters should have been found. The reasons cited4 for the mass assignment are still validj though some of them must now be stated in a more involved way to distinguish between the two 12~hour bismuth activities. 204 To relate Pb204m to Po j a large quantity of polonium was prepared and purified, Bismuth daughters were separated at 4-hour intervals. Each bismuth fraction was allowed to stand for 24 hours; then lead was separated from the bismuth. The yield of 68=minute activity in each sample is listed in Table
III 9 which shows that the polonium ancestor has a 3.8-hour half-life.
6 Ghiorso, Jaffey, Robinsonj and Weissbourdj National Nuclear Energy Series,
Plutonium Project Record 9 Vol. 14Bj "The Transuranium Elements: Research
Papers," Paper No. 16.8 (McGraw-Hill Book Co.j Inc,j New York, 1949), UCRL-640 Revised Page 13
Fig. 2. Pulse analysis showing alpha-particle energies for polonium isotopes. UCRL-640 Revised ·Page 14 ..
o RUN
o RUN 2, I HR. LATER
t. RUN 3, 3 HRS. LATER
.. 300
> I- > I (.) . <{ 200 204 Po.
100
(Mev) MU 830
Fig •. 2 UCRL-640 Revised Page 15
Table III
Fraction 68-min. Pb, counts/min. Observed Calculated
1 4900
2 2200 2350 3 1090 1120
1.5-hr •. PQ 205
A l. 5-hour electron capture activity w:lth associated alpha-particles was resolved from the decay curves of polonium produced by irradiation of lead enriched in mass 204 (27 percent)7 with 37-Mev helium ions in the 60-inch Crocker Laboratory cyclotron (Fig. 3). The predominant product of 37-Mev helium 1,2 ion bombardment of an element in this region is known to be the ( a,3n ) product, which in this case is Po 205. No lighter polonium should be produced in this irradiation, and since the heavier ones are already known, Po205 is the best assignment. Periodic separations of the bismuth daughters from a large amount of mixed. polonium activities at 1.5-hour intervals showed that the yield of longer bismuth periods (i.e., half-lives of the order of a week) indicated a parent with a. half-life of 3 to 4 hours. Since the 9-day Po206 is known to grow the 6.4-day 206 Bi , this clearly indicated a second polonium isotope of short half-life growing a longer bismuth. The decay of the bismuth activities showed that the first separation yielded principally a new activity of 14-day half-life, 7This lead was enriched with a calutron in Berkeley. We are indebted to
Dr. E. H. Huffman, Mr. R. C. Lilly, and Mrs. D. B. Stewart for its purification and to Mr. J. T. Vale for its mass analysis. UCRL-640 Revised Page 16
Fig. 3. Geiger decay curve of polonium fro.m Pb204 with 37-Mev helium ions. A, 5o7-hr. Po207; B, 1.5-hr. Po205.
• UCRL-640 Revised Page 17
HOURS MU 831
Fig;, 3 UCRL-640 Revised Page 18 while the last fractions were almost pure 6.4-day Bi206. After the decay of the Bi206, th~ yield of the 14~day bismuth corresponded to a parent of 1.5 hours (Fig. 4). This is the half-life of Po205, thus the new bismuth activity is also mass 205. Its radiation characteristics will be described presently. The alpha-decay daughter of Po205 was identified in a manner similar to that of Po204. A large quantity of polonium activity was prepared, purified, and the. lead alpha-decay daughter activity separated at 1. 5-hour intervals. After 16 hours, the thallium daughters of the lead were separated from each sample. The thalliunt samples were exclusively the 72-hr. Tl201 , since none of the lighter polonium isotopes were produced in this particular bombardment.
The results are shown in Table IV.
Table IV
Fraction 72-hr. Tl, counts/min. Observed Calculated ------~------·-.. -·- 1 650
2 300 325 3 160 163 4 85 81
These data confirm the assignment of the 1.5-hour polonium to mass 205. 201 Neumann and Perlman3 have established the genetic relation between 8-hr. Pb 201 . 8 . and 72-hr. Tl • Barton, ·Ghiorso, and Perlman s}'x)wm, subsequent to the above experi- ments, that 5.5-hr. At209 decays both to Bi205 and to Po209 , thus confirming again the mass assignments. These decay relations are:
8Barton, Ghiorso, and Perlman, private communication, UCRL-640 Revised Page 19
Fig. 4. Yield of 14.5-day bismuth activity plotted to give half-life of polonium parent.
• UCRL-640 Revised Page 20
> .... 1.5 HR . > t; <(
·3 6 HOURS
MU 832
Fig •. 4 UCRL-640 Revised Page 21
At209 EC p0 209 ? Bi209 4> 5.5 hr. > 200yr. stable la !· p. 205 0 EC. ·. Bi205 EC Pb205 ? Tl205 . > 1.5 hr. > 14 day long stable la Pb201 EC Tl201 EC Hg201 S hr. 72 hr. stable
The energy of the alpha-particles of Po205 was determined as 5.22! 0.10 Mev by following the decay of peaks obtained with the alpha pulse analyzer. 206 9 . The pea~ corresponding to Po (5.22 Mev) decayed about 20 percent 1n four hours. Neighboring peaks due to Po208 and Po210 prevented greater accuracy in the energy estimate. The ratio of electron capture disintegrations to alpha disintegrations, is estimated as 400 by counting both the alpha-part~cles and the gross Geiger count, and making the crude assumption of one Geiger count (at 100 percent geometr,y) per electron capture event.
14. 5-day Bi 205 20 The half-life of Bi205 produced by the decay of Po 5. was found to be 14.5 days (Fig. 5). Aluminum absorption curves showed that about 15 percent of the Geiger counts of this isotope are due to electromagnetic radiation. Lead absorption measurements showed a gamma-ray of 1.7 Mev (half-thickness 14.5 g/cm2). With a beta spectrometer we .found oonversion electrons corresponding to 1.84 Mev for this g~a-ray, as well as others for gamma-rays of 431, 527, 550 and 746 Kev. The existing data are insufficient to establish the decay scheme.
9A. Ghiorso, private communication. UCRL-640 Revised Page 22
Fig. 5. Decay of 14.5-day Bi 205. UCRL-640 Revised Page 23
100
60 80 100 DAYS - MU 8 35
Fig. 5 UCRL-640 Revised Page 24
47-min. Po 203 Repetition of the experiment described above--separation of the 12-hour bismuth daughters from a large sample of polonium at regular intervals-- indicated that two polonium parents were producing 12-hour bismuth. The signifi- cant difference from earlier experiments was that the polonium was purified fairly rapidly, so that the daughter separations were start'ed within an hour of the end of the bombardment. The data indicated another polonium isotope 204 . . 203 of a shorter half-life than Po . It must be at mass 203, since 52-.hr. Pb ·· appeared in the decay curves of the first bismuth fractions, but not in later ones. The half-life of the Po203 was determined by preparing a large sample of polonium, purifying quickly after bombardment, ·and separating the bismuth daughters at half-hour intervals. The bismuth was allowed to decay for 24 • hours, then. the lead activity produced by bismuth decay was separated from the bismuth fraction. Since the lead daughters were allowed to grow into each bismuth fraction for the same period, the activity of 52-hr. Pb203 grown in was directly proportional to the initial activity of Bi203 in each fraction.
The data plotted in Fig. 6 determine the half-life as 47 ! 5 minutes.
52-min. Po202 and 95~min. Bi202 In addition to the other bismuth activities mentioned above, we observed a 95-minute activity among the bismuth daughters (Fig. 7) of polonium produced - in bombardments at fairly high energy (protons of more than 70 Mev on bismuth,
or helium ions of more than 120 Mev on lead)~ The yield of this 95-minute activity as a fUnction of separation time showed that its polonium parent has
a half-life of 52~ 5 minutes (Fig. 8). UCRL-640 Revised Page 25
Fig. 6. Yield of 52-hr. Pb203 showing a 47-minute polonium parent • .UCRL-640. Revised Page 26
>- 47 MIN 1- >. 1- u <(
-~~------~----~~~--~~----~~--~ 60 120 180 240
MINUTES MU 836 164161
Fig. 6 UCRL-640 Revised Page 27
Fig. 7. Decay of bismuth daughter activities. A, 12-hr. Bi203 and 12-hr. Bi204; B, 95-min. Bi202 • UCRL-640 Revised Page 28
-.
>- 1- > 1- (.) 100 2 4 6 10 12 -HOURS MU 8~4 Fig~ 7 . .. UCRL-640 Revised Page 29 202 Fig~ 8. Yield of 95-min, Bi showing a 52-minute polonium parent . UCRL-640 Revised. Page 30 10 > 1- (.) <( ·• 0 .. 160 240 MINUTES Mt) 8:33 Fig. 8 '<. '. UCRL-640 Revised Page 31 If this polonium parent were the Po203 described above, then the new bismuth activity would be an isomer of Bi 203. In all attempts to milk Pb20.3 from this bismuth the yield corresponded to a bismuth parent of 12-hour half-life. However, this result does not rule out isomerism; if the ratio of isomeric transition to electron capture tor the upper state is equal to (1 -~)/1 , where ! and 1 are the respective half-lives of the upper and 2 1 1 2 lower states, the daughter is produced at a rate corresponding exactly to decay of a single parent of half-life t • -2 The 95-minute activity was not detected in bismuth formed by irradiation of lead, containing 2J percent Pb 204, with 18-Mev deuterons from the 60-inch 7 cyclotron. Since the (d,Jn) reaction is known to take place with high yield 2 204 at this energy on bismuth, it is expected to do the same Qn Pb • Bismuth of mass 203 was formed in this experiment, because lead separated from bismuth after a period of growth showed Pb20.3 in good yield. Therefore, it was presumed that the 95-minute activity is not Bi203. If the assignment were 201 or 200, known lead and thallium activities should pave been found as daughters. Thus 202 is the best assignment. Additional evidence for the assignment to mass 202 was obtained from an excitation experiment, which at the same. time ~dentified the alpha-particles of 5.59 ! 0.0.3 Mev energy (Fig. 2) with Po202 • The electrostatically deflected proton beam from the 184-inch e,yclotron was directed on a target composed of copper absorbers with bismuth foils interposed. From each foil the polonium was separated and purified. Pulse analysis of an aliquot of the polonium gave the yield,. of the 48-minute alpha-activity. After a period of about two hotirs, the bismuth daughters were separated from the remainder of the polonium • •< Decay measurements on an aliquot of this bismuth gave the relative yield of <,.' UCRL-640 Revised Page 32 . ~2 Bi 202 and thereby the yield of Po • The remainder of the bismuth was allowed to decay approximately a d~, after which lead was separated and purified. The relative yield of Pb203, and thereby of Po2°3, was determined by following decay of this lead fraction. The resulting data are listed in Table V. Table V Total Proton Relative Yielda Absorber Energy!' 95-min. Bi Pb203 Thickness (Mev) (g/cm2) 82.76 (90) 0.93 0.12 84.74 (80) 1.01 0.22 85.68 (75) 1.10 0.22 86.59 (70) 1.14 0.36 88.34 (60) 4.5 ain arbitrar,y units, proportional to the yield of 48-minute alpha activity in the same foil. buncertain because the initial energy is not known exactly, and because of straggling in the' absorbers. cNot detected. From these data it is clear that the alpha-particles belong to the parent of the 95-minute bismuth, and that the mass assignment is less than 203. The variation of the yields from one foil to another is not presented here because of uncertain chemical yields in the isolation of the polonium; however, these data are in agreement with a mass difference of one unit for the 95-minute bismuth and Pb203. UCRL-640 Revised Page 33 No alpha-particles were observed which could be attributed to Po203. However, if the electron capture to alpha disintegration ratio were ten times larger than that of Po202, it is doubtful if the alpha-particles could have been observed. From the energies of the alpha-particles of the other polonium isotopes, a reasonable prediction is that the alphas of Po203 have an energy of 5.4 ± 0.1 Mev. No reliable data are available concerning the energies of the electrons and gamma-rays of Bi202, since our samples were never pure. No positrons nor alpha-particles were found associated with it. If the alpha-dec~y ~o electron capture decay ratio is less than 10-5, it is doubtful that alpha-decay would have been detected. Identification of the lead alpha-decay products of Po2°2 anq,, Po2°3, or of the thallium daughters of these leads, failed for lack of sufficient radio chemical purity of the daughter samples. The half-life of Pb198 was determined by an experiment described below. Thus the decay of Po202 is presumed to be: 202 p0 202 EC Bi202 EC Pb202 ? n2o2 EC Hg :> ~ => .... 52 min. :> 95 min. long 12 day stable la l? Pbl98 EC nl98 EC Hgl98 25 min. !> 1.8 hr. !> stable The long-lived Pb202 is still undetected. · 25-min. Pb198 The half-life of Pbl98 was estimated by establishing a genetic relation between the 1.8-hr. nl98 and its lead parent. This was done in the manner described before; a large sample of lead activities was prepared by bombarding UCRL-640 Revised Page 34 thallium with protons of 120 Mev, the lead separated and purified, and the thallium daughters separated at 30-minute intervals. The resolved yields of the 1.8-hour thallium show a half-life of 25 ! 10 minutes for its lead parent. Neumann and Perlman3 have observed directly a lead activity which had an I apparent half-life of 25 minutes, in agreement with our value. ACKNOWLEDGMENTS It is a pleasure for the authors to acknowledge the assistance of Dr. H. M. Neumann in the beta-spectrometric study of Bi205, of Mr. Albert Ghiorso with the alpha pulse analyzer, and of Mr. J. T. Vale, Mr. G. B. Rossi and the cyclotron crews in making the irradiations on the 184-inch and 60-inch u cyclotrons. We wish to thank Professors I. Perlman and G. T. Seaberg for helpful discussions in the course of this work • • ..